1. Field of the Invention
This invention relates to a device for restoring image signals.
2. Description of the Related Art
The image signal restoring devices which have heretofore been known include an electronic still camera system. The electronic still camera system is arranged to record a still image signal on, for example, a magnetic disc and to reproduce the recorded still image signal. FIG. 1 of the accompanying drawings shows the frequency spectrum of a recording still image signal obtained by the electronic still camera system. Referring to FIG. 1, a luminance signal component Y is frequency-modulated after an emphasis process. A chrominance signal component C is frequency-modulated to a low frequency band after two color-difference signals R-Y and B-Y are converted into a line sequential state and is then subjected to an emphasis process. The recording image signal is obtained by frequency-multiplexing the luminance signal component Y and the chrominance signal component C. One field amount of the recording image signal is recorded in each of a plurality of concentric tracks formed on a magnetic disc.
FIG. 2 shows the still image signal transmitting and receiving device of the electronic still camera system arranged to reproduce a still image signal which is recorded on the magnetic disc in the above-stated manner and to transmit the reproduced signal, for example, over a telephone line or the like. In the still image signal transmitting and receiving device of FIG. 2, the magnetic disc 33 having the record of the still image signal is rotated by a motor 34 in accordance with a field period. A magnetic head 32 reproduces the recorded signal from the magnetic disc 33 which is thus rotated. The reproduced signal is sufficiently amplified by a pre-amplifier 31 and is, after that, supplied to a high-pass filter (hereinafter referred to as HPF) 23 to have a luminance signal component separated therefrom. The luminance signal component is then restored to a luminance signal by a frequency demodulator 22 and a deemphasis circuit 21. Further, the output of the pre-amplifier 31 is also supplied to a low-pass filter (hereinafter referred to as LPF) 30 to have a chrominance signal component separated by the LPF 30. The chrominance signal component is then restored to a color-difference line-sequential signal by a frequency demodulator 29 and a deemphasis circuit 28.
In order to transmit over a telephone line the still image signal which is restored and reproduced in the above-stated manner, the reproduced still image signal must be time-base-expanded and band-compressed by temporarily taking it into a digital memory and by reading it out at a speed which is slower than a speed at which it is taken into the digital memory. To meet this, in transmitting the still image signal, the connecting positions of switches SW10 and SW9 are set on their sides "b" respectively in accordance with an instruction given from a system controller which is not shown. This allows the reproduced luminance signal to be supplied as it is to a color-difference-to-RGB conversion matrix circuit 15. Meanwhile, the reproduced color-difference line-sequential signal is supplied to a switch SW13 which is connected to its one side "b". The switch SW13 supplies the color-difference line-sequential signal to a line-simultaneous conversion circuit which consists of a delay (DL) circuit 26 for delaying the input for one horizontal scanning period (hereinafter referred to as H) and switches SW11 and SW12. The line-sequential signal is then line-simultaneous-converted into two different color-difference signals. The two color-difference signals thus obtained are supplied also to the color-difference-to-RGB conversion matrix circuit 15.
The connecting position of each of the switches SW11 and SW12 is arranged to be on one side "b" thereof when a color-difference signal R-Y is received and to be on the other side "a" when a color-difference signal B-Y is received. These signals R-Y and B-Y are simultaneously output from these switches SW11 and SW12. The signals R-Y and B-Y are supplied to an addition type 1H comb-filter which consists of 1H delay circuits (DLs) 24 and 25 and adders 24a and 25a. The filter performs an averaging process on these signals R-Y and B-Y before they are supplied to the color-difference-to-RGB conversion matrix circuit 15.
The color-difference-to-RGB conversion matrix circuit 15 forms R (red), G (green) and B (blue) color signals by using the luminance signal and the two color-difference signals supplied to the circuit 15. The R, G and B signals are supplied to analog-to-digital (hereinafter referred to as A/D) converters 9, 10 and 11. Each of the A/D converters 9, 10 and 11 then samples and converts the input signal into a digital signal of eight bits per sample. The digital signals thus obtained are respectively taken into an R memory 6, a G memory 7 and a B memory 8 in blocks of filed signals. Meanwhile, a synchronizing (hereinafter referred to as sync) signal is separated by a sync signal separation circuit 18 from the luminance signal which is output from the switch SW9. The sync signal thus separated is supplied to a memory control circuit 16 via a switch SW5 which is connected to one side "b" thereof. Then, in synchronism with the sync signal supplied, the memory control circuit 16 designates the writing addresses of the digital memories 6, 7 and 8. The action of writing these digital signals into these memories is thus controlled.
After one field amount of each of these digital signals (or data) is written into each of the digital memories 6, 7 and 8, the data is read out from the memory in synchronism with a sufficiently low-speed clock signal for the purpose of transmitting it via the telephone line. The data thus read out is supplied via a switch SW1 to a digital-to-analog (hereinafter referred to as D/A) converter 4. The D/A converter 4 converts the input data into an analog signal. The analog signal is then supplied to a modulator 2 to be converted into a signal form suited for the telephone line. The output of the modulator 2 is sent to the telephone line 1 via a switch SW14 which is connected to one side "a" thereof. Further, a frequency modulator or an amplitude modulator is employed, for example, as the modulator 2. The switch SW1 is provided for the purpose of supplying the R, G and B signals stored in the digital memories 6, 7 and 8 to the D/A converter 4 in the form of a frame-sequential digital signal. The connecting position of this switch SW1 is changed from one side thereof over to another every time one field amount of data is caused to be output from the digital memories 6, 7 and 8 by the system controller which is not shown.
In receiving a still image signal which is supplied via the telephone line 1, on the other hand, the switch SW14 is connected to one side "b" thereof to receive the input still image signal. The input signal is demodulated by a demodulator 3. The demodulated image signal is A/D-converted by an A/D converter 5. The output of the A/D converter 5 is then supplied to the digital memories 6, 7 and 8.
Since the input signal from the telephone line 1 is a frame-sequential signal consisting of R, G and B signals, the connecting position of the above-stated switch SW15 is changed from one contact over to another according to these signals by the system controller which is not shown. Meanwhile, the connecting positions of the switches SW2, SW3 and SW4 are on their sides "b" at that time. Further, if the input signal transmitted is a field still image signal, a computing operation is performed, at the same time, at each of the digital memories 6, 7 and 8 to form an interpolating field still image signal for a field image signal not transmitted.
Further, in cases where a still image signal which is thus restored from the transmitted input signal is to be displayed on an external monitor device or to be printed out by a printer, the restored signal must be output to the outside in the form of R, G and B signals. In outputting the transmitted input signal to the outside in this manner, the data stored in the digital memories 6, 7 and 8 is first read out in the following manner: A sync signal is generated by a sync signal generator (SSG) 17. The sync signal is supplied to a memory control circuit 16 via a switch SW5 which is connected to one side "a" thereof. The memory control circuit 16 then causes the data to be read out from the digital memories 6, 7 and 8 at a high speed in such a way as to make the data into the original still image signal. The data thus read out is supplied to D/A converters 12, 13 and 14, which convert the input data into analog R, G and B signals respectively. These analog R, G and B signals are then output via 75-ohm driving circuits 35, 36 and 37 to the applicable external device. Further, the sync signal generated by the sync signal generator 17 is also supplied to the external device via the switch SW5 and a 75-ohm driving circuit 38.
As described above, even in a case where the transmitted input signal is a field still image signal, the interpolating process computing operation performed at the digital memories enables the system to form and produce a frame still image signal without any skew.
In transmitting the still image signal reproduced from the magnetic disc over the telephone line in accordance with the procedures described in the foregoing, it is occasionally required to confirm the contents of the signal by means of a monitor device or the like before the signal is transmitted. On that occasion, if the reproduced still image signal is a field still image signal, an interpolating field still image signal must be formed with a known skew compensating process applied thereto. However, the skew compensating process would require an excessively long period of time if it is carried out by performing a computing process at the digital memories. To avoid this, the conventional system has been arranged to perform the skew compensating process by means of an analog circuit which consists of 0.5H analog delay circuits 19 and 27 and a 1H analog delay circuit 20 which are shown in FIG. 2.
Further details of the above-stated reproducing processes are as follows: The reproduced luminance signal output from the deemphasis circuit 21 is supplied as it is to the adder 20a via the side "b" of the switch SW10 while it is supplied also to the 1H analog delay (DL) circuit 20. At the adder 20a, an average value of a signal which is delayed for a 1-H period by the 1H analog delay circuit 20 and a signal which is not delayed is computed. By this, an interpolating luminance signal is obtained and is supplied to the side "a" of the switch SW10. An interpolating action is performed on the luminance signal by changing the connecting position of the switch SW10 between its sides "a" and "b" for every field period. The interpolated luminance signal is thus output from the switch SW10. The position of the switch SW10 stays on the side "a" for a vertical equalizing pulse interval during the above-stated operation.
The luminance signal output from the switch SW10 undergoes a skew compensating process performed jointly by an 0.5H analog delay circuit 19 and a switch SW9. More specifically, the connecting position of the switch SW9 is on one side "a" thereof while the switch SW10 is on its side "b". This allows the luminance signal from the switch SW10 to be output via the 0.5H analog delay circuit 19. The position of the switch SW9 is on the other side "b" while that of the switch SW10 is on its side "a" to allow the luminance signal from the switch SW10 to be output as it is. Further, in the operation described, the luminance signal which is allowed to pass through both the 0.5H analog delay circuit 19 and the 1H analog delay circuit 20 is never output. Further, an interlaced frame luminance signal is output from the switch SW9 by always keeping the connecting position of the switch SW9 on the side "b" thereof during the period of the vertical equalizing pulse interval.
Further, to perform a skew compensating process also for the color-difference signals, a 0.5H analog delay circuit 27 and a switch SW13 are likewise operated to accomplish the skew compensating process, in the same manner as in the case of the luminance signal. After that, the color-difference signals are subjected to a line-simultaneous conversion process before it is output.
An interlaced frame still image signal is formed in the above-stated manner, including two color-difference signals and a luminance signal. The frame still image signal is supplied to the color-difference-to-RGB conversion matrix circuit 15 to be converted into R, G and B signals. The R, G and B signals are output through the switches SW6, SW7 and SW8 and the 75-ohm driving circuits 35, 36 and 37. At that time, the switches SW6, SW7 and SW8 are on their sides "a". Meanwhile, the switch SW5 is on its side "b" to allow the sync signal which is separated by the sync signal separation circuit 18 to be also output through the 75-ohm driving circuit 38.
However, the conventional device which is arranged as described in the foregoing has presented the following problems: For carrying out the interpolating and skew compensating actions on the luminance signal and the line-simultaneous converting and skew compensating actions on the color-difference signals in reproducing the still image signal recorded on the magnetic disc, the conventional device uses an analog delay element such as a CCD (charge-coupled device) in general. However, the use of the CCD necessitates highly sensitive adjustment for correcting the level of the delayed signal. In addition to that, a temperature drift is apt to take place to bring about a flicker due to level variations in the output still image signal.